Method and system for improving usable life of memory devices using vector processing

Abstract

A method, system and apparatus for improving the useful life of non-volatile memory devices such as flash memory. The present wear-leveling technique advantageously improves the overall useful life of a flash memory device by strategically moving inactive data (data that has been infrequently modified in the recent past) to the memory blocks that have experienced the most wear since the device began operation and by strategically moving active data to the memory blocks that have experienced the least wear. In order to efficiently process and track data activity and block wear, vectors of block-descriptor pointers are maintained. One vector is sorted in decreasing order of overall block erase / write activity (block-wear indicator), whereas the other vector is sorted in increasing order of the number of times a block has been erased since the last wear-leveling event occurred (activity indicator for the data stored in the block). The activity levels of the data and the wear levels of the blocks are then easily compared and otherwise processed using pointers into these vectors to allow for more efficient processing than previous techniques used for wear leveling.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to data storage devices, and in particular relates to improved techniques for using and managing non-volatile memory storage devices to enhance lifetime characteristics of such devices.[0003]2. Background of the Invention[0004]Memory devices are a key component in today's computer systems, allowing high-speed access to retrieve and store data. A certain type of memory device known as non-volatile memory maintains the correct data value stored in the device even when power is removed from the device. Because non-volatile memory devices are more expensive than volatile memory devices (which lose data when removed from a power source), their use in a computer system is typically limited to situations where a need exists to maintain data even when power is removed or lost. For example, configuration settings for a computer are typically stored in non-volatile memory so that the system properly ...

AI Technical Summary

Benefits of technology

[0014]The present invention is a method, system and apparatus for improving the useful life of non-volatile memory devices such as flash memory. The present wear-leveling technique advantageously improves the overall useful life of a flash memory device by strategically moving inactive logical blocks (logical blocks that have been infrequently modified in the recent past) to the physical blocks that have experienced the most wear (the greatest number of erasures) since the device began operation and by strategically moving active logical blocks to the physical blocks that have experienced the least wear. To track the activity of both physical blocks and logical blocks, a block descriptor is maintained for each physical block. One component of the block descriptor, denoted by n, is the total number of times the physical block has been erased since the device began operation; this component measures the cumulative wear on the physical block. Another component of the block descriptor, denoted by Δn, is the number of times the physical block has been erased since the most recent wear-leveling event; since logical blocks are moved only during wear-leveling events, this component measures the activity of the logical block that is currently stored in the physical block.

[0015]The information processing required to support wear leveling employs two vectors that contain the same elements but in different orders. The elements of each vector are pointers to all the block descriptors. One vector is sorted in decreasing order of the cumulative wear (n), while the other vector is sorted in increasing order of recent activity (Δn). These two vectors contain all the information that is required for effective wear leveling. Furthermore, this information is organized in such a way that the most promising opportunities for wear leveling can easily be identified by comparing elements of the two vectors as these vectors are traversed (using pointers into the vectors) in accordance with a suitable algorithm.

Problems solved by technology

Because non-volatile memory devices are more expensive than volatile memory devices (which lose data when removed from a power source), their use in a computer system is typically limited to situations where a need exists to maintain data even when power is removed or lost.

Certain non-volatile memory devices such as EEPROM age over time, and thus have a limit on the number of times they can be re-programmed.

However, when these devices are used in applications having a large amount of input and output operations—such as when used in conjunction with or in lieu of a disk or tape subsystem—it does become an issue.

The problem is to devise a wear-leveling algorithm that can be relied upon to distribute wear more evenly, with acceptable overheads, under conditions that can be expected to arise in practice.

1. The choice of logical blocks to be moved in a wear-leveling operation is based on the cumulative wear of a given physical block, which depends on all the logical blocks that have resided in that physical block since the device began operation; the activity level of the logical block currently stored in the physical block is thereby obscured, leading to suboptimal decisions. For example, it would be desirable to populate the most worn physical block with the least active logical block, but the logical block currently residing in the least worn physical block (i.e., the physical block with the smallest cumulative wear) is not necessarily the least active logical block.

2. Wear-leveling operations can be counterproductive if they are performed too frequently: a logical block that is frequently modified and has been moved from a physical block with high wear to a physical block with low wear may be moved back to the physical block with high wear.

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